In recent years, growing sophistication of terrorist threats to homeland and abroad makes awareness-of chemical and biological substances, including chemical and environmental toxins and biohazardous materials, of great importance. This awareness brings about a need for the accurate sensing and monitoring of these types of substances, especially those that may be present as air-borne particles/molecules and are dangerous to humans. The desired level of sensing sensitivity is that which provides for the accurate sensing of various chemical and biological substances at levels that are considered potentially dangerous, while preventing false alarms for levels of materials that are not considered potentially dangerous.
Biohazardous materials are defined as those substances that are naturally occurring in nature such as SARS, influenza, smallpox, anthrax, plague or the like. Sensing and monitoring the presence of these types of materials provides awareness in both naturally occurring situations and when intentionally used in a hazardous manner. The intentional use of biohazardous materials is referred to as the use of biological agents, such as one or more organisms, or one or more toxins derived from living organisms, against people, animals, or crops. In addition, sensing and monitoring of various chemical and environmental toxins, including pesticides and herbicides, is needed. While these man-made chemical and environmental toxins may provide beneficial qualities when used properly, these toxins may become chemical agents if wrongly used.
Many types of sensors have been developed to detect a variety of chemical and environmental toxins and biohazardous materials, but there are currently more toxic substances and hazardous biological materials that need to be sensed than there are suitably sensitive and discriminating sensors. The most common current method of sensing and monitoring chemical and environmental toxins and biohazardous materials is accomplished using mass spectrometers. This method of detecting substances typically uses relatively large monitored equipment that is not typically amenable to situations where portable monitoring devices are needed. For instance, mass spectrometers are commonly used in an airport setting where items passing through security may be swabbed and the presence of controlled or banned substances is sensed. The mass spectrometer used is typically a permanent, or semi-permanent, sensing unit that is monitored by security personnel.
Of growing interest is the use of optical sensing devices to sense and monitor substances of interest. In many instances, these devices include a waveguide in which a beam of light is propagated. The optical characteristics of the device are influenced by variations at the surface of the waveguide, such as a change in the total reflection. Other types of optical sensors are based on the use of a sensing optical fiber in which the fiber serves as an optical transmission line that, in conjunction with a sensor device, detects the presence of various substances based on light transmission loss. These optical fibers provide for sensing along the length of the fiber.
In existing concepts, an optical sensor operates by transmitting light of a wavelength spectrum from a light source via a fiber to a sensing section, a sensor or sensor array. The light is then directed from the sensing section or sensor(s) to a tunable filter driven by a waveform generator which is scanned to detect the intensity of light within each wavelength band of the of the source light wavelength spectrum. A portion of the light, in the spectrum corresponding to a subset of wavelengths within the spectrum, i.e. a channel, is affected by the sensed condition or sensed substance in the sensor or sensing section. The peak of intensity of the light coming from the sensing section, or sensor(s) for each channel is detected and a digital pulse representative of the peak of the detected light in each channel is generated. The digital pulses are converted to a value which is proportional to the intensity of light in a channel centered at a particular wavelength. Using a model of the sensor's relationship of intensity versus wavelength for measurement of a particular parameter, a measurement value based on this parameter can be made. For example, a fiber optical sensing section may be used, with a fiber having an increased loss of a particular wavelength band in the presence of a hazardous gas. In this case, there will be less light in that particular wavelength band in the presence of the gas, and a dip in intensity will be observed at the detector at this wavelength, but not across the whole source spectrum. In this way, a measurement of the gas concentration can be made.
All-purpose, multi-gas optical sensor systems have been found to be very expensive, primarily because of the cost associated with the various light sources needed to illuminate a sensor or sensor array with light of the appropriate spectral bandwidth; that is, containing the large range of wavelengths needed to stimulate transitions in all the substances of interest. In addition, conventional optical fibers cannot be used for the sensing section without major modification, in that the light's electric field does not extend out into the environment, meaning that does it not interact significantly with the environment in which the sensor resides. Because of the light source power and spectral requirements, and because of the filter requirements, the cost, weight and volume are significant in prior art systems, which can limit the use of these systems in portable sensor applications or other environments in which a light weight or compact monitoring system is needed or desired, but a highly accurate sensor is required.
Accordingly, there exists a need for an improved optical fiber sensor system which avoids these prior art deficiencies and would be useful in a user friendly system such as a system which monitors chemical and environmental toxins and biohazardous materials. This invention relates to an optical sensor and method of using the sensor for the sensing and monitoring of chemical and environmental toxins and biohazardous materials in an atmosphere. In addition, there is a need for an improved optical fiber sensor system that could be used in the area of homeland security and battlefield security. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.